Underwater surfaces are subject to fouling by a complex fouling community involving many different organisms. The primary fouling organisms include bacteria, diatoms, and algal spores. Bacteria are the first organisms to attach and likely lay down a conditioning film on the surface. Their attachment is followed by that of diatoms and algal spores. Hard fouling organisms appear in the second stage of fouling and include barnacles, algae, mollusks, tube worms, and sponges.
Tributyltin (TBT) and organotin derivatives like tributyltin oxide (TBTO) have been the most effective antifouling biocides in use to date. However, their use has many severe environmental consequences. These antifoulants are highly toxic to non-target aquatic organisms, they bioaccumulate with accumulation factors as high as 10,000, their degradation products are highly toxic, and they are believed to be endocrine disruptors.
Since the application of organotins was banned in 2003 the search for alternatives has begun. Copper oxide is the commonly used alternative at present, but since it is not very effective against algae and diatoms, it is usually used in conjunction with an organic booster biocide to help control these organisms. (Omae, I. “Organotin antifouling paints and their alternatives” Applied Organometallic Chemistry 2003, 17, 81-105.)
Based on the impact of TBT and its derivatives on the near-shore marine environment, the environmental fate of new marine antifouling biocides is subject to regulatory scrutiny. This poses a key dilemma for the invention of new antifouling coatings. On the one hand, the active agents must degrade to non-toxic byproducts at a sufficiently rapid rate that prevents their accumulation in the environment. On the other hand, an effective antifouling coating must have a long effective lifetime in contact with seawater. A currently used antifoulant Sea-Nine™ (4,5-dichloro-2-octyl-4-isothiazolin-3-one) meets the former criterion, rapidly degrading via ring opening reactions. However, Sea-Nine is simply dispersed in a suitable support matrix such as paint and leaches into the environment at a rate that depends upon diffusion through the supporting matrix. A constant effective concentration is therefore difficult to sustain over a prolonged period. (Jacobson, A. H.; Willingham, G. L. “Sea-nine antifoulant: an environmentally acceptable alternative to organotin antifoulants” Science of the Total Environment 2000, 258, 103-100.)
Research on poly-substituted guanidinium salts has shown that they are able to inhibit growth of algal communities. In laboratory experiments, the exposed algal communities recover and grow once the active agent has degraded to a low concentration. However, in order to use the salts as anti-fouling agents, they have to be formulated in mixtures that can be used to coat the surface to be protected. Attempts to employ the mixtures in the field demonstrated that the effective lifetime of the mixtures was limited, with the lifetime dependent upon the temperature of the marine environment. (Fyles, T. M.; Rowe, R. D. “Microbiocidal properties of poly-substituted guanidinium salts” U.S. Pat. No. 6,518,309 B1 2003).
Known slow-release marine coating formulations employ a wide variety of strategies. The principal strategy exploits the physical ablation of the surface by motion of water past the treated surface, so-called “self-polishing” surfaces. This continually exposes fresh surface, either for diffusive release of a soluble component such as Sea-Nine, or the exposure of an insoluble component such as copper oxide. Such a strategy requires significant water movement and is ineffective on slow moving or fixed submarine structures. An alternative strategy relies on a chemical reaction with the seawater to release a continuous supply of the active agent which is initially attached to the coating matrix. This was particularly effective for esters of TBT which hydrolyzed at acceptable rates to release tribuyltin oxide. (Omae, op cit.) Such hydrolysis reactions can be used in conjunction with the self-polishing strategy to release insoluble materials such as copper oxide, but are not appropriate for biocides such as Sea-Nine which readily diffuse through the matrix, and themselves undergo hydrolysis reactions at appreciable rates. Release of ionic components such as copper, zinc, or alkylammonium salts via acid-base reactions with a supporting matrix has also been described. Such processes are subject to the particular chemical properties of the ions released and do not offer a general method. (Iwamura, G.; Konno, E.; Shoji, A.; Yokoyama, Y.; Tatsuno, Y.; Shimizu, S. “Antifouling coatings containing difficultly hydrolysable acrylic polymers, Japanese patent JP01129077 A2 1989; Arimoto, Y.; Hayashi, S.; Rakutani, K; Shoida, Y. “Durable marine antifouling paints”, Japanese patent JP 03252462 A2 1991).
There is therefore a need for antifouling formulations containing environmentally acceptable agents that can be formulated to slowly release an effective concentration of the active agent. It is an object of the present invention to overcome the deficiencies in the prior art.